(1) Field of the Invention
The present invention relates to artificial snow in a granule or an aggregate form and a novel method for making the same, and more particularly to artificial snow in a granule or in an aggregate form suitable for skiing, both of which have a particle structure similar to that of compact natural snow in granule form and a method for making the same. The method of the present invention comprises absorbing water into a super absorbent polymer in granule form which can retain the granule form after absorption of water without sticking each other, and freezing the water-swollen super absorbent polymer.
Further, the present invention relates to an improver of snow quality to control snow conditions freely so as to give artificial snow suitable for skiing by blending it with natural snow, artificial snow made by a snow machine or icy snow made by shattering ice blocks to fragments.
(2) Description of the Prior Art
(Natural snow)
In the recent years, there have been marked decreases in snowfall accumulation and as a result, considerable hindrance of the opening of many ski areas has been occasioned. Furthermore, it is necessary to pack the snow on a slope by a tracksetter several times in order to prepare a well-groomed packed slope for sliding on skis because newly fallen natural snow is too soft and is not suitable for smooth sliding on skis. On the other hand, the snow condition of a tracksetter-groomed trail is not suitable for a ski racing competition and the trail has to be groomed by a method comprising of treading the slope underfoot, then spraying water on spread snow, and the like.
The snow conditions of runs prepared by such methods are very susceptible to influences from the outside air temperature, and change from compact snow into corn snow with the passage of time. Said change is caused by the fact that sublimation and condensation of water molecules of snow cause a change in snow granules.
It is not easy for skiers to slide smoothly on skis on a slope covered by corn snow, and thus frequent grooming, for example, breaking the crust snow is indispensable. These attempts, however, still gave poor simulation of natural snow conditions.
Artificial snow
In recent years, installation of snow machines has become popular in many ski areas to prolong the overall skiing season. Conventional snow machines fall into two groups consisting mainly of "gun" type or "fan" type machines.
A method for making artificial snow by use of those snow machines comprises atomizing compressed water into a subzero atmosphere to make fine ice particles with the aid of adiabatic expansion of compressed air or cooling air. The artificial snow thus produced contains 10% by weight or more of water, and has a density of about 0.3 to about 0.4 g/cm.sup.3 with a mechanical strength of less than about 1 kg/cm.sup.2. The slope covered by such artificial snow is still not skiable without being packed. Such artificial snow changes its quality more quickly than natural snow and in certain cases, forms corn snow having an average outer diameter of about 1 to 5 mm in a few days. As mentioned above, corn snow is troublesome to skiers and hence a measure similar to the aforesaid is required to resolve the situation.
The methods for producing artificial snow and apparatus are disclosed in U.S. Pat. Nos. 3,716,190; 3,010,680; 3,301,485; 3,298,612; 3,761,020; 3,774,843; 3,774,842; 3,945,567; 4,004,732; 4,083,492; 4,105,161; 3,733,029; Australian Patent Applications Nos. 77956/75 and 12534/83.
Other art references disclose methods comprising supplying water-particles into the a current of air from a fan (U.S. Pat. Nos. 3,780,598; 2,968,164), freezing the particles with the addition of ice crystals (U.S. Pat. No. 3,598,478) or bacteria (U.S. Pat. No. 4,200,228) as a nucleus to accelerate the formation of snow flakes or snow crystals.
International Application PCT/AU86/00158 (International Publication WO86/07373) discloses a method for making artificial snow which comprises mixing water with an absorbent material which swells upon absorbing water, and exposing thereafter the resulting water-swollen material to the air followed by freezing. The snow produced thereby has a density of about 0.4 to 0.9 g/cm.sup.3 with a mechanical strength of about 10 to several 100 kg/cm.sup.2. These ranges show that density and mechanical strength of the snow vary widely depending on the conditions of snow being exposed and frozen. Such product has a stiff, fine ice form, i.e. an "Eisbahn" in quality, rather than snow. Therefore, when artificial snow suitable for use to cover ski slopes is made from the water-swellable material alone, addition of a surfactant, regulation of particle sizes and water content, and frequent grooming of slopes are necessary to prevent an excessive agglomeration of frozen particles. Such snow is rated among the most difficult snow to handle for ski hill operators. U.S. Pat. Nos. 3,247,171; 3,022,279; 3,022,280; 3,251,194; GB Patent Application No. 2127005; and Australian Patent No. 464077 disclose the aforesaid absorbent materials.
As snow conditions on ski slopes are susceptible to influences from the weather, indoor artificial skiing slopes have become popular in recent years. A snow cover for the indoor ski slopes is also made by use of the above-mentioned artificial snow, artificial ice granules, snow fragments or artificial snow made from a water absorbent material which swells upon absorbing water with which the indoor slope is covered or coated. In this instance, the aforesaid problems still remain unsolved. Another indoor ski slope is constituted by a method comprising covering the slope with a material in paste form which is a mixture of a absorbent polymer with water (blending ratio=about 1/80 to about 1/100), freezing the cover throughout the slope, and subsequently grooming the frozen cover surface by scraping so as to make artificial snow. In addition to the aforesaid problems, such slope has an inherent problem that pole planting is not so easy in this type of snow, because there is an ice layer like "Eisbahn" underneath the artificial snow.
(Comparison of a granule structure of natural snow with artificial snow)
Natural fresh snow fallen on ski slopes contains over 90% by volume of a void space and is not suitable for skiing. Such fresh snow compacted by itself or groomed by a tracksetter is the most appropriate snow for skiing.
Accordingly, the microscope photographs of fresh, compact and artificial snow were compared one another.
Photograph No. 1 showing granules like stars is a microscope photograph of a structure of natural fresh snow in which several parts of the six branches of a snow crystal are narrow so as to form small ice beads, some of the adjacent two beads aggregating together to form a bottle gourd.
Photograph No. 2 is a microscope photograph of a structure of natural compact snow which shows no snow granules like stars any more but many bottle gourds formed by aggregation of most of the adjacent two ice beads so as to provide a snow aggregate. It is observed that there is still a void space in a relatively small amount but it distributes uniformly all over among the ice phases of these bottle gourds.
Photograph No. 3 is a microscope photograph of a structure of aforesaid artificial snow in which frozen water-swollen polymer granules (black regions) and ice constitute a harmonious whole and the polymer granules can not be distinguished from the ice phase in which water-swollen polymer granules themselves in bare state appear on the surfaces of snow granules and most of the void spaces are unevenly distributed to form large voids (white regions). This type of snow structure has no ice beads and bottle gourds which exist in the snow structure of compact natural snow.
Photograph No. 4 and 5 are microscope photographs of structures of artificial snow made by freezing conventional absorbent polymers after absorption of water. Photograph No. 4 shows the structure of a conventional water-swollen polymer frozen in the original granule form in a similar way shown in Photograph No. 3. Photograph No. 5 shows a structure of an another conventional water-swollen polymer in granule form (spherical regions) frozen in the original swollen form and it seems to have a somewhat different structure from that of the snow shown in Photograph No. 3, but in fact there are many scars (black dendritic regions) on the surface of the polymer granule caused by breaking of the ice surrounding the polymer granule and the bare surfaces of the polymer uncovered with ice appear on the surfaces of snow granules. There is little void space in the structure and the structure is also entirely different from that one of compact natural snow.
Such artificial snow having structures shown in photographs Nos. 3 to 5 gave poor suitability for sliding on skis in on-the-spot evaluation. The estimated reasons why these structures of artificial snow give poor results are summarized as follows:
(a) Such artificial snow having entirely different structures from that one of packed natural snow may provide only rough and fine icy snow, corn snow or snow like "Eisbahn" wherein frozen polymer and ice constitute a harmonious whole and most of the small void spaces are unevenly distributed so as to give a large void space, or said structure includes little void space and broken polymer granules or granules themselves in bare appearing state on the surfaces of snow granules. PA1 (b) When the absorbent polymer granules in bare state appear on the surfaces of snow granules, friction may increase as a result of direct contact of soles of skis with the absorbent polymer. PA1 (a) Packing procedures are indispensable to prepare well-groomed trails for skiing. PA1 (b) It is not easy to make snow having a desired density and strength, i.e. it is difficult to freely adjust the snow conditions to the one that matches skier's abilities, performaces or preferences. PA1 (c) Snow conditions vary widely with the passage of time. Therefore, it is difficult to maintain the snow quality in good conditions on ski slopes for a relatively long time period. PA1 (d) Investment and maintenance costs for artificial slope are too expensive. PA1 (e) Artificial snow produced by freezing water and a water-swellable material forms an ice block. Therefore, the process for crushing the ice block is essential. PA1 (a) said polymer can retain its original granule form after absorption of water without sticking each other; PA1 (b) said polymer has a deionized-water absorbability of about 30 to 500 times the weight of the dry polymer; PA1 (c) said polymer has an average particle size of about 20 to 500 .mu.m before water absorption; and PA1 (d) said polymer has an average particle size of about 0.05 to 5 mm after water absorption. PA1 (a) absorbing water into a super absorbent polymer in granule form which can retain its granule form after absorption of water without sticking to each other, and has a deiontzed-water absorbability of about 30 to 500 times the weight of the dry polymer, an average particle size of about 20 to 500 .mu.m before water absorption so as to give a water-swollen polymer having an average particle size of about 0.05 to 5 mm; and PA1 (b) freezing the water-swollen super absorbent polymer-by mixing it with a coolant.
Because of this, artificial snow in granule or aggregate form having the structure similar to that of compact snow shown in Photograph No. 2 may provide good snow quality for skiing.
Some of the problems associated with natural snow or artificial snow used to cover ski slopes include the following.